In the modern telecommunications world, voice communications continue to be a popular mode of communication, but new services like video telephony, high speed data and short message services continue to expand on existing services. The arrival of new telecommunications services generates new requirements for telecommunications networks. New telecommunications techniques (transfer modes) are required and offer possible advantages compared to existing techniques. Traditional transfer modes for wired communications are circuit switching, familiar in classical telephone services, and packet switching, familiar in telegraphy and modern short message service and data systems.
Asynchronous transfer mode (ATM) is a mode of fast packet switching which allows systems to operate at a much higher rate than traditional packet switching systems. Features which characterizes ATM communications are: the ability for asynchronous operations between a sender clock and a receiver clock; transmission "cells" of pre-defined sizes; operation at a basic frame rate, with all transmissions being at integer multiples or devisors of the frame rate; and addressing carried out in a fixed size header (that is not by time, frame position or other fixed characteristic). ATM communication is sometimes also referred to as asynchronous time division (ATD) communications.
ATM communication has proven useful in high-value point-to-point land-line communication, for example, satellite links and undersea cables. ATM allows multiple simultaneous circuits, sometimes referred to as virtual circuits (VCs), to be established from end to end along the link.
Attention is turned to the use of ATM for wireless or radio communications. There is, for example, a need for wireless users to have access to wired ATM networks and existing ATM systems such as multi-media applications need a wireless platform providing multi-media support. It is also recognized that systems such as universal mobile telephone systems (UMTS) and wireless local area networks (LANs) cannot meet all future data user needs. Efforts to date have been in the use of ATM in the wireless extension of fixed infrastructure systems, such as LANs and integrated service data network (ISDN).
For private land mobile networks and ad-hoc wireless local area networks, circuit-switched frequency-division multiple access (FDMA) with or without time division multiple access (TDMA), as well as code division multiple access (CDMA) continue to be the available transfer modes. Each of these transfer modes has its advantages and disadvantages in different circumstances and the various modes are generally incompatible with each other.
Referring to FIG. 1, two sets of communicating units are illustrated, each set functioning as an independent network. These networks are illustrated as networks 10 and 11 comprising a first set (set A) of nodes or units as well as a second set (set B) of nodes or units. Four units of the first set are shown as units 12, 14, 15 and 16. One unit of the second set is shown labeled 13. Each unit may be referred to as "terminal" or a "node". Each unit 12, 13, 14, 15, 16 may be a fixed or portable data terminal, or a fixed or portable two-way radio, or indeed a video telephone or other communicating unit. The units 12, 13, 14, 15, 16 will simply be referred to hereafter as "radio units". Each set of radio units consists of two or more radio units communicating with each other. While any member of one set may interfere with the transmissions of one or more members of the other set (and any further sets not illustrated), it is possible, indeed probable, that any given radio unit may not be able to directly receive the transmissions of such other radio units that it may interfere with or that may interfere with it, thereby making conventional methods for avoiding interference as listen-before-talk (carrier sense) ineffective.
Discussions to date in the field of wireless ATM systems have focused on centralized systems such as wireless LAN systems. For ad-hoc communications, traditional schemes such as slotted ALOHA are favored.
International patent application No. WO95/01020 describes a wireless local area network with distributed control and describes an "ad hoc" embodiment with no access points. Communication takes place through protocol data units (PDUs). Each directed PDU consists of four "frames" (which in this context are packets): a request-to-send (RTS) packet, a clear-to-send (CTS) packet, a data packet and an acknowledgemnet (ACK) packet. A general header format is described for asynchronous service PDUs. Each packet type has a unique format and the different packet types have different lengths. Thus an RTS packet is 13 bytes long and a data packet is up to 598 bytes long. By using listen-before-talk and an RTS/CTS exchange, the effect of collisions is minimized because collisions are limited to the short RTS packet and avoided in the longer data packet. Queuing theory shows, however, that access times are not minimized when packets of different lengths are contending on the channel.
An embodiment is described in WO95/01020 which is a wireless LAN with access points supporting time-bounded (e.g. voice) service, it is described that the payload of a PDU may be an ATM cell or multiple ATM cells and that each access point manages bandwidth in its area and controls the timing of connections and initiates all the time-bounded PDUs. Once a connection is granted, the access point unit sends the first PDU at a time coordinated with other ongoing connections and after the first PDU, subsequent PDUs for that connection are sent at fixed intervals and each time-bounded PDU is a two-packet exchange. This is wasteful of bandwidth and suffers from slow receive-transmit switching times. Different schemes are described for managing the bandwidth either to keep maximum size gaps available for asynchronous traffic or to minimize latency for the asynchronous service. A GAPTIME field in all time bounded packets specifies when the next PDU for the connection will be sent. GAPTIME is used to reserve bandwidth for subsequent packets of the same voice connection. This "reserve ahead" mechanism prevents other nodes from contending for the network during the transmission of time bounded packets for the ongoing voice connection, but such a "reserve ahead" mechanism is very inconvenient, as it depends on listening to and decoding activity on the channel, which is power-consuming and unreliable.
In the ad hoc embodiment described in WO95/01020, all mobile units transmit announce packets. Each unit hops and scans during each hop so as to receive announce packets from other units. A hop frequency of choice is identified and mobile units change their hop frequencies to the hop frequency of choice and follow the same hop sequence. Transmission of announce packets by all units is also inconvenient and wasteful of channel capacity and battery power.
Although ATM communications are considered to have the potential for improved operating characteristics, such as increased throughput, reduced access time and reduced queuing times, no ad-hoc wireless ATM system has yet been devised with a robust protocol which can realize these perceived advantages.
There is a need for an improved wireless communication system, particularly one that is suitable for ad-hoc communications.
A preferred embodiment of the invention is now described, by way of example only, with reference to the drawings.
______________________________________ AAL ATM adaptation layer; ALOHA A contention scheme involving random access attempts; ATM Asynchronous transfer mode; ATMH ATM header; CAC Channel access control; CAQ Contention access queue; CRC Cyclical redundancy check; OQ Out-of-sequence queue; RAQ Reserved access queue; RSSI Received signal strength indication; SAMA "Simple" ATM multiple access; SDU Service data unit; VCI Virtual circuit identifier; VPI Virtual path identifier. ______________________________________